Decomposition of the dose conversion factor based on fluence spectra of secondary charged particles: Application to lateral dose profiles in photon fields.

Abstract

The dose conversion factor plays an important role in the dosimetry by enabling the absorbed dose in the sensitive volume of a detector to be converted into the absorbed dose in the surrounding medium (in most cases water). The purpose of this paper is to demonstrate that a specific fluence-based approach for the decomposition of the dose conversion factor is in particular useful for the interpretation of the influences of detector properties on measurements under nonreference conditions. Data for the dose conversion factor and secondary fluence spectra were obtained by the Monte Carlo method. The calculation of the secondary charged particle fluence (electrons and positrons) in the sensitive detector volume was imbedded into the code for the calculation of absorbed dose in the detector. The decomposition method into subfactors is based on the use of these fluence data applied to a stepwise transition from the dose at the point of measurement next to a pure water detector and finally to the fully simulated detector geometry. Each subfactor is obtained as a ratio, at which the stopping power only is different in the numerator and the denominator or at which the fluence only is different in the numerator and the denominator. This method was applied at photon dose profiles obtained in water at different radiation qualities and with various detectors of cylindrical type. The resulting subfactors can be well identified as a stopping power ratio and as perturbation factors each reflecting particular detector properties. Two of them (f1 and f4 ) are equivalent with perturbation factors which have already been introduced by other authors previously. These are the volume perturbation factor and the extracameral perturbation factor. Subfactor f2 denoted as medium perturbation factor was found to resemble the density perturbation factor. Results obtained for the volume perturbation factor applied to dose profiles measured with cylindrical detectors confirm that the volume effect can be well described by a convolution of the true profile in water with a Gaussian kernel. It was found that the sigma parameter depends on the cylinder radius only and amounts almost exactly to half of its value. The medium perturbation factor strongly depends on the density of the detector medium. For an air-filled detector, the influence of the air again can be described by a Gauss convolution, however, with a less good agreement. For detectors with a density of the cavity medium larger than that of water, for instance, for a diamond detector, it was found that there is a tendency of compensation between the volume averaging effect and the medium effect. The fluence-based decomposition of the dose conversion factor leads to a fluence-based formulation of perturbation factors, referred to as volume, medium, and extracameral perturbation factor. These factors offer useful explanations for the behavior of detectors in nonreference conditions. An example was given for cylindrical detectors at dose profile measurements.